1. Field of the Invention
The invention relates to variable capacitors generally, and more specifically, to variable capacitors adapted for high-pressure gas filling.
2. Description of the Prior Art
Variable capacitors have been variously constructed to increase their voltage and current handling ratings. In high-powered, radio-frequency equipment, vacuum capacitors have been extensively employed. Such devices are normally enclosed within a glass or ceramic shell or enclosure, thereby to provide electrical insulation as well as hermetic sealing. U.S. Pat. Nos. 2,511,338; 2,740,926; 2,920,255; 3,257,590; and 3,270,259 (the latter providing a fluid cooling feature within the evacuated envelope) are representative of the vacuum capacitor prior art.
Inert fluids have also been used in the dielectric space between the capacitor plate sets in some types of sealed variable capacitors. The device of U.S. Pat. No. 2,867,758, although involving a substantially different combination than that of the present invention, does refer to the use of an atmosphere of inert gas in a sealed container. Various gasses have been used in such devices. One specialty gas known in the trade as sulphur hexaflouride (SF.sub.6) is extensively used as an inert gas filler for various types of sealed electrical components, including variable capacitors of the type to which the present invention applies.
It is a known characteristic of the aforementioned SF.sub.6 gas that its dielectric strength is a direct function of the pressure of the gas in a given vessel. In view of the various design trade-offs encountered in connection with variable capacitor design, it is very advantageous to be in a position to provide increased dielectric strength within the gaps between the plate sets of the capacitor. Accordingly, either higher voltage ratings may be achieved for the same plate spacing, or alternatively, for the same voltage rating a higher capacitance rating can be obtained by closer plate spacing with attendant reduction in size, weight and cost of the completed device.
The exploitation of the advantages of SF.sub.6 gas according to the foregoing, may require that it be under a number of atmospheres of pressure. Considering, for example, a typical pressure of 100 psig in a more-or-less typical capacitor assembly having five inch enclosure (housing) diameter, it will be realized that relatively large hoop stresses are generated in the housing shell. It also follows that typical prior art enclosure materials, such as the glasses or ceramics, are unsuitable for the application. Not only is the pressure differential from inside the enclosure to the outside atmosphere greater than the maximum ever encountered in a typical vacuum capacitor structure, but also the said hoop stress is a tension force, rather than the compressive mode stress, resulting from a maximum one atmosphere pressure differential (inside to outside), as encountered in the shell of a well known prior art vacuum capacitor structure. It is well known that the typical prior art enclosure materials such as the aforementioned glasses or ceramics, have much lower inherent strength in tension, as compared to the compression mode. Thus, the use of these prior art dielectric materials for the enclosure of a pressure filled vessel severely limits the pressure which may be used and introduces the danger of explosion and fragmentation of the enclosure material with attendant potential for equipment damage and personnel injury.
The manner in which the present invention deals with the disadvantages of the prior art to provide a unique combination overcoming the prior art difficulties and providing certain additional ancillary advantages will be understood as this description proceeds.